Double helix microwave structure for coupling a microwave magnetic field from a first to a second region

ABSTRACT

The device comprises a first helix, a second helix and a stem structure interconnecting the first and second helices. The first helix of the device is inserted into a resonant cavity of a microwave spectrometer to couple the microwave magnetic field therein, via the stem structure, to the second helix mounted about a sample placed outside the cavity. After interaction of the electromagnetic field with the sample, the second helix reflects the electromagnetic field back into the cavity, via the stem structure and the first helix.

United States Patent Inventors Ronald M. Tlu'bault Franklin Park. Ill: Dale W. Schutt. So. Bend. Ind.

Appl, No. 864,480

Filed Oct. 7, I969 Patented Aug. 10. I971 Assignee The United States of America as represented by the United States Atomic Energy Commission DOUBLE HELIX MICROWAVE STRUCTURE FOR COUPLING A MICROWAVE MAGNETIC FIELD FROM A FIRST TO A SECOND REGION 1 Claim, 3 Drawing Figs.

U.S. CI 333/24 R, 324/05 A, 324/585 C, 333/84 R, 333/97 R Int. Cl IIOIp 5/00 Field of Search 324/05, 58, 53.5, 0.5 A, 0.5 E; 343/895; 315/36; 333/24, 31,

[56] References Cited UNITED STATES PATENTS 2,762,950 9/l956 Lund 333/83 X 2,860,280 ll/I9S8 McArthur 333/83 X OTHER REFERENCES Pearlman et al., Characteristics of Traveling Wave Helices in ESR Spectrometers, The Review of Scientific Instruments, Vol 38, No. 9, Sept. 1967, 333- 24.

Primary Examiner- Eli Lieberman Assistant Examiner- Paul L. Gensler Attorney-Roland A. Anderson DOUBLE HELIX MICROWAVE STRUCTURE FOR COUPLING A MICROWAVE MAGNETIC FIELD FROM A FIRST TO A SECOND REGION CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to electromagnetic waveguides, and more particularly to a waveguide for coupling a microwave magnetic field in a microwave cavity to a location outside the cavity.

In microwave spectroscopy information on atomic or molecular properties of certain materials is obtained through measurement of the frequencies at which interaction takes place between a microwave field and a sample of the material. In atypical device for making these measurements, the sample is placed in a resonant cavity receiving an electromagnetic field from a microwave source. At particular frequencies of the microwave source energy exchanges occur which cause a change in the energy of the electromagnetic field in the cavity. This change in energy of the electromagnetic field is then converted into a low frequency electrical signal proportional thereto by a microwave detector monitoring the microwave field in the cavity. Among the disadvantages of such devices are their sensitivity to frequency changes in the microwave source, since the microwave source cannot be kept tuned to the peak of the cavity Q-curve to suppress phase shift effects in the electromagnetic field. Also, since the sample is placed in a cavity it is difficult to apply external modulating fields or optical radiation to the sample.

It is therefore an object of the present invention to provide means for coupling a microwave field in a cavity to a location outside the cavity.

It is another object of the present invention to provide means for coupling a microwave magnetic field in a resonant cavity of a microwave spectrometer to a sample placed outside the cavity.

It is still another object of the present invention to provide a helical slow wave structure which is readilyadaptable for use with a conventional microwave spectrometer.

SUMMARY OF THE INVENTION Briefly, the device of the present invention comprises a first helically wound wire structure, a second helically wound wire structure, and a stem structure interconnecting the first and second structures.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the invention will best be obtained from consideration of the accompanying drawings in which:

FIG. 1 illustrates a preferred embodiment of the present invention;

FIG. 2 illustrates an exemplary use of the device of the present invention with a conventional microwave spectrometer; and

FIG. 3 is a 3-dimensional view of a microwave cavity of the spectrometer of FIG. 2 and illustrates the relative position of the device of the present invention with respect to the microwave field therein.

PREFERRED EMBODIMENT OF THE INVENTION Referring now to FIG. 1, there is shown a first helical or spiral structure 2, a second helical or spiral structure 4, and a stem structure 6. Structures 2 and 4 comprise a helically wound low resistance wire such as gold-plated silver wire or copper wire. The stemstructure 6, also of low resistance wire,

interconnects the structures 2 and 4 such that the axis of the helical structures 2 and 4 lies along the stem structure 6. The stem structure 6 makes a sharp right angle bend into the first turn of the helical structures 2 and 4. Preferably, the double helix structure comprising the helical structures 2 and 4 and the stem structure 6 are constructed as a unit from a single low resistance wire. Helical structure 4 is short circuited at one end thereof by soldering the last two turns together to form a closed loop 8. Helical structure 2 is left open circuited. To an observer positioned between the helical structures 2 and 4 and facing each helical structure, in turn, along the axis of the stem structure 6, the turns of structure 2 are wound counterclockwise and the turns of structure 4 are wound clockwise.

With reference to FIG. 2, the function and operation of the double helix structure are best understood by considering its use with a microwave cavity such as a resonant cavity of a conventional electron spin resonance spectrometer. A microwave bridge or so-called magic tee 10 includes a first arm 14 connected to the output of a microwave source 12, a second arm 16 connected to a termination load 18, a third arm 20 connected to a resonant rectangular cavity 22, and a fourth arm 24 connected, via a microwave detector 26, to a recorder 28.

Cavity 22 includes an opening or sample port 34 through which, in the conventional use of the spectrometer, a sample to be tested is placed inside the cavity. In the present use of the spectrometer with the double helix structure, a sample 32 to be tested, such as a liquid, is placed in an envelope 30, such as a quartz tube. The double helix structure of FIG. 1 is also mounted within the envelope 30 so that one helix 4 surrounds at least part of the sample 32. The envelope 30 is inserted into cavity 22, via the sample port 34 to position helix 2 within the cavity and the stern structure 6 and helix 4 outside the cavity. The envelope 30 thus acts as a sample container as well as a support for the double helix structure.

A pair of field modulation coils 35 and 36 are positioned on either side of the sample 32 to produce a varying magnetic field therethrough in response to the varying output current of a conventional scanning unit 38 connected to the pair of coils.

Referring now to FIG. 3, and with cavity 22 selected to resonate in the TE mode for purposes of describing a typical operation of the double helix structure with cavity 22, the associated electric and magnetic fields therein are respectively represented by lines 40 perpendicular to a sidewall 23 of the rectangular cavity and closed lines 42 parallel to the sidewall 23. With the sample port 34 located directly below the center of the cavity 22, the helical structure 2, when inserted therethrough, is thus placed in the maximum microwave magnetic field in the cavity since both magnetic fields represented by closed lines 42 have the same direction along a line through the center of the cavity and the center of the sample port.

Before operation, the spectrometer is adjusted in the conventional manner by adjusting the load 18 and the cavity 22 with the double helix structure inserted such that an electromagnetic wave from the source 12, via arm 14, is completely absorbed by the load 18 and the cavity 22 after division thereof at the junction of the bridge 10. Thus in the balanced condition of the bridge 10, the detector 26 produces no output.

In operation of the spectrometer with the double helix structure, electromagnetic waves from the source 12 pass through arm 14 and divide equally between arms 16 and 20. The electromagnetic waves traveling through arm 20 are received by the cavity 22 which forces the electromagnetic waves to resonate in the TE mode having the characteristic magnetic and electric field pattern as shown FIG. 3. The microwave magnetic field through the helical structure 2 in the cavity 22 induces a current therein, which current, via stern structure 6 produces a microwave magnetic field through helical structure 4 and hence through the sample 32. The induced current is reflected at the closed loop 8 of helical structure 4 thereby reflecting the microwave magnetic field through helical structure 4 back into the cavity 22.

Scanning unit 38 produces a slowly varying magnetic field through the sample 32 perpendicular to the microwave magnetic field produced by the helical structure 4. At a certain value of the slowly varying magnetic field produced by the pair of coils 35 and 36 interaction between the microwave magnetic field produced by helical structure 4 and the sample 32 takes place in the form of electron spin resonance in the sample 32 which causes a change in the magnitude of the reflected microwave magnetic field into cavity 22. This change in the reflected field into cavity 22 unbalances the magic tee and detector 26 produces a signal representative of the electromagnetic field change in cavity 22 which is recorded by recorder 28.

The optimum dimensions of the structures 2, 4, and 6 for best performance of the double helix device depend on the particular frequency region of the microwave field in which the device is used. In one embodiment for use in an electron resonance spectrometer operating in the 10.5 Gl-lz. region, the present device was constructed as a unit from No. AWG silver wire. The helical structures 2 and 4 were constructed with an outer diameter of 5 mm., a pitch of 20 threads per inch, and a length of L3 cm. The length of the stem structure 6 was 8 cm.

The double helix device can, of course, also be used with cavities in which the electromagnetic waves resonate in modes other than the TE mode selected for the description of the operation of the above-described embodiment of the invention.

Since the present double helix device allows placement of the sample outside the cavity, it will be appreciated that the sample may be readily subjected to light or other radiation if so desired, as well as magnetic fields, as shown above. It is to be further noted that the present double helix structure is a broadband device. For low temperature or cryogenic work the frequency shifts and the Q (quality factor) of the resonance may change drastically for the resonant cavity which is a narrow band device. These undesirable effects can be overcome by using the double helix device, which responds only slowly to temperature variations, and by thermally isolating the cavi- Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments different from the embodiment illustrated. Accordingly, the scope of the protection afforded the invention should not be limited to the particular embodiment illustrated in the drawings and described above, but should be determined only in accordance with the appended claim.

The embodiments of the invention in which we claim an exclusive property or privilege are defined as follows:

1. A device for coupling a microwave magnetic field confined to a first region to a second region spatially separated from said first region, comprising:

a first helical structure disposed within said first region in coupled relationship with said microwave magnetic field;

a second like helical structure aligned with said first helical structure to have a common axis therewith and disposed within said second region;

said first and second helical structures being wound in 0pposite directional sense about said common axis relative a point therebetween; and

a stem structure interconnecting said first and second helical structures coupling said microwave magnetic field therebetween. 

1. A device for coupling a microwave magnetic field confined to a first region to a second region spatially separated from said first region, comprising: a first helical structure disposed within said first regiOn in coupled relationship with said microwave magnetic field; a second like helical structure aligned with said first helical structure to have a common axis therewith and disposed within said second region; said first and second helical structures being wound in opposite directional sense about said common axis relative a point therebetween; and a stem structure interconnecting said first and second helical structures coupling said microwave magnetic field therebetween. 